This invention relates to interrupted flight screw presses, and more particularly to the worm and collar arrangement used in such presses.
Screw presses generally include a cage with inlet and outlet at opposite ends, defining a pressing chamber through which a rotatable screw assembly extends.
Each worm includes an annular body with an integral helical flight which extends circumferentially around all or almost all of the body, such as a worm where the flight extends circumferentially about 340.degree. around the body. The worms and flights are spaced axially by collars also mounted on the shaft, and some means is provided for resisting rotation of material within the chamber to cause axial flow of material from each worm to the next successive worm. Typically, breaker bars or lugs project inwardly toward the collars for this purpose.
The screw assembly comprises a series of axially spaced worms mounted on a shaft and the wear pattern varies as the internal pressure increases and the fluids contained within the pressed solids drains away. Frequently, only the worms in the heavy wear locations need be replaced after an extended running period.
In operation, the shaft is supported and driven from one end and lateral components of forces acting on the cantilever mounted screw assembly can cause wear on the outer surface of the worms. In an arrangement where a cantilevered shaft is driven from the feed end, the wear is especially pronounced on the last few worms of the screw assembly since that end of the assembly is subject to greater lateral deflection. In many presses the extremely high pressure generated near the discharge end of the screw press coupled with the higher friction from a compacted material from which most of the fluids have been drained causes more rapid erosion of the flights and bodies of the worms than in the feed and/or low pressure end of the screw press. When the screw press is used for expressing liquids from material containing abrasives, for example, sugar cane bagasse having particles of sand therein, the wear is still further accelerated.
To minimize wear on the worms, hard coating on the outer surface of the flights and bodies of the worms has been employed, particularly on the worms closest to the discharge end of the press. Also cage contractions have been provided (as in U.S. Pat. No. 3,093,605) to make it easier to obtain access to the screw assembly to replace worm parts. But, it is still necessary to withdraw the worms and collars over the end of the shaft in order to replace worn or damaged worms.
In presses used for the drying or dewatering of synthetic rubber materials, it is sometimes necessary to provide for pressure changes (increase or decrease) beyond the range available through adjustment of the press core. Also, some presses have a drive connection at the discharge end of the press, (as in U.S. Pat. No. 3,276,354), and the tendency is for heavier wear to occur in the region of the discharge. Thus, whether it is desirable to change a number of worms and/or collars, or only one or more near the discharge end, it is desirable to minimize down-time of the press for such changes.
Also, presses used in some applications where corrosive liquids or materials are involved are provided with stainless steel parts, for example, shafts, worms, and worm bodies, etc. If it is necessary to slide a stainless steel worm body along a stainless steel shaft, of any appreciable difference, there is a tendency for these parts to gall; therefore, it is advantageous to minimize the amount of negative longitudinal movement between these parts necessary for replacement of the worm bodies.